1. Field of the Invention
The present invention relates to antennas. More specifically, the present invention relates to quadrifilar helix antennas.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
The Global Positioning System (GPS) provides accurate position information in three dimensions (latitude, longitude, altitude). Position location is facilitated by a constellation of satellites. Each GPS satellite continuously transmits precise time and position data. GPS receivers read signals transmitted from three or more satellites and calculate the user's position based on the distance therefrom. In addition to position information, other navigation information may be calculated including, range, bearing to destination, speed and course over ground, velocity, estimated time of arrival and cross track error. The accuracy of the calculation is dependent on the quality of the signal detected from the satellite. Hence, the system requires a sufficiently accurate receiver and antenna arrangement. Specifically, the antenna must be small and portable with an omnidirectional beam pattern broad enough to detect signals from satellites located anywhere in the hemisphere. For this purpose, the quadrifilar helix antenna has been found to be well suited.
As discussed in Antenna Engineering Handbook, by Richard C. Johnson and Henry Jasik, pp. 13-19 through 13-21 (1984) a quadrifilar helix (or volute) antenna is a circularly polarized antenna having four orthogonal fractional-turn (one fourth to one turn) helixes excited in phase quadrature. Each helix is balun-fed at the top, and the helical arms are wires or metallic strips (typically four in number) of resonant length (1=m.lambda./4, m=1, 2, 3, . . . ) wound on a small diameter with a large pitch angle. This antenna is well suited for various applications requiring a wide hemispherical beam pattern over a relatively narrow frequency range.
In accordance with conventional wisdom, quadrifilar helix antennas are constructed of several pieces (e.g. 13) typically soldered by hand at numerous joints. The antennas are typically mass produced by unskilled labor. As a result, quadrifilar helix antennas constructed in accordance with conventional teachings are expensive to fabricate, nonrepeatable in design and therefore require hand tuning. In particular, conventional quadrifilar antennas have a coax feed which has a varied distance between the inside diameter and outside diameter to match the 50 ohm typical input impedance to 30 ohm typical feed output impedance for optimum power transfer into the antenna elements. This requires machining and hand assembly which complicates the design and increases the cost of construction.
Thus, there is a need in the art for a quadrifilar antenna design that allows for low construction and testing costs.